1. Field of the Invention
This invention generally relates to computer systems and more specifically to apparatus for enabling the exchange of data to a computer interface from a peripheral device.
2. Description of the Prior Art
Digital computers generally operate in conjunction with external devices, called "peripherals" that feed input data into the computers and accept output data from them. Because peripherals may be electromechanical devices and the computers are electrical devices, the data as it is stored on each may be in a different form in order to ensure reliability for the data transfer. For example, peripheral devices having encoded NRZ data oftentimes miss interpretation since it is difficult to isolate the transition of binary ones and binary zeros without providing the time frame of reference. It has been found appropriate to use a data coding scheme in which the message contains clocking information so that reliability in the information retrieving process is ensured.
This combination of data with clocking information is called a self-clocking system. Two problems arise in magnetic recording system: (1) peak shifting or, as it is also known, phase jitter; and (2) recording frequency of the data. When data is stored on the peripheral it may be located at a place different from where it should be ideally located. This phenomenon is known as peak shifting or phase jitter and is an inherent property of the magnetic interaction. If the peak shifting becomes too great, the original data may actually be misinterpreted or lost.
In order to have proper transfer of data, it is required that the data pulses be precisely located on the peripheral devices not only to provide proper reading/writing of the data but also to eliminate the deskewing problems which occur when the data pulses are transmitted through delay lines in peripheral modules. Thus, the greater precision in locating and transferring the pulses, the lesser problems with reading and writing the data.
When data is stored or retrieved, it must be coordinated with timing pulses so that the appropriate time to sense the data is known. Thus, it is necessary that the data pulses be placed in the same slot as the timing pulses. This is known as clock synchronization. Depending on the speed of the device, the recording frequency of the data may be changed.
One prior art system as shown in U.S. Pat. No. 3,614,635 achieves time relocation for a fixed recording frequency by stretching the duration of the actual data pulses a fixed amount. These stretched data pulses are combined with phase difference pulses derived from the timing pulses to increment or decrement the frequency of a VCO. This system provides identical delay circuits to accentuate the difference between the clock pulses and the data pulses so that frequency changes to the provider of the clock pulses (oscillator) are rapidly implemented. Moreover, this system provides for relocation of the data, however, when the data is late it is relocated to the peak shifted time whereas in the case when the data is early it is relocated to the clock time. As a result, this system suffers from the disadvantage that precise timing for exchange of the data does not occur. This increases the deskewing problem.
The above system, like other previous systems, is also frequency dependent and is not able to be used over a range of frequencies. Thus, these systems hold the clock frequencies to a given value. If this frequency changes, the feedback control system does not work.
Finally, the above system is not able to be applied to various data rate schemes. For example, with Miller coded data it is necessary to determine whether the data transition occurs at a bit cell or bit boundary since this transition identifies whether the data is a binary one or a binary zero. This further increases the need for accurate correlation between time and data since slight variations may cause data misinterpretation.